1. Field of the invention
The present invention concerns an actuator comprising a direct current motor, a non-reversible gearbox and a speed reducer driving an output shaft.
The invention applies advantageously to autopilot control of the tail rotor of a helicopter to control the bearing (yaw control).
2. Description of the prior art
Known yaw flight controls in helicopters comprise a linkage connecting the rudder bar controlled by the pilot to the tail rotor. In the specific case of yaw control the linkage comprises a friction device the main function of which is to define an anchor or mean control point as the last "memorized" position of the linkage subsequent to action on the rudder bar by the pilot. In autopilot mode the bearing of the helicopter is maintained by a parallel or trim actuator which, in addition to the components listed above, comprises a clutch between the non-reversible gearbox and the speed reducer plus means for sensing the position of the output shaft which controls the movement applied by the actuator to a non-fixed point of the linkage. The actuator components are housed in a dust-proof casing. In one prior art implementation the yaw flight controls comprise said friction device in a parallel arrangement and a force sensor in series with the rudder bar.
When the autopilot is engaged the bearing is sensed and compared by the autopilot computer to a set point value. If the actual bearing and the set point value are different, an error signal is applied to the power supply of the motor until the aircraft is returned to the required bearing by the action of the trim actuator.
Because of the presence of the friction device the last position of the output shaft is fixed. When the helicopter pilot wishes to resume manual control he applies to the rudder bar an action detected by the force sensor which if the force is above a threshold F.sub.B cuts off the power supply to the clutch of the actuator, so disengaging the autopilot. Provided that the threshold F.sub.B is less than the sum of the friction F.sub.e applied by the friction device and the non-localized friction F.sub.t of the flight control linkage, the pilot resumes yaw control in the last position of the trim actuator before it was disengaged. Because the pilot must be able to overcome the friction F.sub.e relatively easily when flying manually, the friction F.sub.e must be below a comfort threshold F.sub.c.
The value of F.sub.e must therefore satisfy the condition: EQU F.sub.B -F.sub.t &lt;F.sub.e &lt;F.sub.c ( 1)
Using the conventional dry friction technique in which two parts rub on each other, the friction F.sub.e is relatively inaccurate and unreliable. It requires frequent adjustment if the relationship (1) is to be always satisfied.
A major drawback of prior art yaw trim actuators is that in autopilot mode the actuator motor must overcome at all times the resisting torque induced by friction at the output shaft, so that it is necessary to use a powerful motor with a short service life.
One object of the invention is to overcome the technical problem of providing an actuator of the kind described previously which, by virtue of an advantageous configuration of the friction device within the actuator system, has the advantage that in autopilot mode said friction does not resist drive movement of the motor but, with the autopilot disengaged and manual control resumed, defines the yaw flight control anchor point as the last memorized position of the actuator.